The proposed research deals with the chemical phase of radiation biology and more specifically is concerned with the action of radicals on nucleoproteins. It attempts to translate advances in radiation chemistry and molecular biology into a basic understanding of the effects of radiation on biologically significant macromolecules. The current objective is the elucidation of the initial chemical changes produced in chromatin on gamma-irradiation. In pursuit of this aim, chromatin isolated from cultured Chinese hamster cells, a sub-unit of chromatin consisting of a DNA-core histone complex, and free DNA will be examined. These entities will be irradiated under controlled environmental conditions so as to evaluate the effects of individual aqueous intermediates: Oh., eaq-, H. and O2-. Specifically, the following shall be investigated: 1) modifications in DNA, in particular assessing the influence of the associated chromosomal proteins by comparing the damage to DNA in the three entities; DNA will be examined further for double- and single-strand breaks, for base damage, and for interstrand crosslinks. 2) damage in the chromosomal proteins, especially the histones, a) by measuring the destruction of specific amino acid residues, and b) by determining changes in their size and conformation. 3) modifications in the chromatin structure, a) by further examination of the radiation-induced DNA-histone adducts to identify the sites of crosslinking, b) by measuring changes in the accessibility of the DNA using probes, including nucleases which produce discrete sub-units of chromatin and DNA-intercalating agents. It is hoped that these studies will clarify the nature of the initial chemical damage to chromatin in vivo and the mechanism of how proteins modify radical attack upon the genetic apparatus.